Staudinger Predation by four fish predators on two squid species on the Northwest Atlantic continental shelf 



613 



planation is that the opportunistic foraging strategy of 

 goosefish is more sensitive to variation in relative and 

 overall abundances of prey and results in erratic food 

 habits over various time scales. 



Predator-prey behavior 



Differences in the amount of squid consumed among 

 species, among predator sizes, and seasonal periods 

 indicate that aspects of predator and prey behavior must 

 be playing a pivotal role in influencing the susceptibility 

 of squid to predation. Seasonal inshore-offshore habitat 

 use and diurnal vertical migrations are two mechanisms 

 that may act to mediate predator-prey encounter rates. 

 Other behaviors that influence encounter rates are the 

 times of day that each predator is actively hunting 

 and the areas of the water column being searched. Of 

 the four predators evaluated, seasonal movements by 

 summer flounder are most similar to the migration pat- 

 terns of longfin squid. Summer flounder are primarily 

 daytime feeders and spend the majority of their time on 

 or near the seafloor (Packer et al., 1999). Conversely, 

 silver hake are a demersal species known to be active 

 hunters primarily in the late afternoon and evening 

 (Bowman, 1984). Large vertical movements in the water 

 column are not customary for silver hake (Bigelow and 

 Schroeder, 1953), therefore nighttime feeding limits 

 interactions during seasons when squid exhibit strong 

 diurnal migrations. Bluefish are the only predator in this 

 study considered to be pelagic and that actively pursue 

 prey throughout the water column. Although bluefish 

 have been shown to forage primarily during the day 

 (Juanes and Conover, 1994), it is possible that there is a 

 seasonal shift in depth at which bluefish forage (Bigelow 

 and Schroeder, 1953). A shift from feeding at depth in 

 the late spring and summer to feeding in the surface 

 waters during fall may explain, in part, seasonal dif- 

 ferences in importance of squid to bluefish because few 

 (if any) squid are present in the upper water column 

 during daylight. 



In general, daytime foraging behavior, combined with 

 positioning in the demersal environment, create the op- 

 timal setting for regular encounters between squid and 

 fish predators and explain differences in the proportion 

 or even presence of squid in the diet between species 

 with similar geographical ranges. 



Implications for management 



Since the late 1980s, squid harvests have increased 

 substantially during the winter. Offshore harvests have 

 been, on average, three times greater than inshore 

 catches between October and March (Cadrin'M. The 

 combination of increased fishing pressure and elevated 

 predation rates concentrated within a single season may 



not be sustainable for a species, such as squid, with a 

 life span of less than a year and that has little overlap 

 between generations (Pierce and Guerra, 1994). If not 

 properly accounted for, this increase in total mortality 

 on squid during the winter could affect the number of 

 squid surviving to spawn during spring and summer and 

 limit available biomass for predators such as bluefish 

 and summer flounder. 



Another area of concern stems from potential in- 

 creases in top-down pressure from recovering predator 

 populations. Regionally, management has emphasized 

 age-truncation of predators as one of the most seri- 

 ous factors affecting summer flounder and silver hake 

 populations. The age classes of summer flounder (age-2) 

 (Terceiro'-) and silver hake (age-3) (Brodziak'^) tar- 

 geted for expansion in their respective management 

 plans, correspond to the lengths and associated ages 

 identified in the present study as being the most vora- 

 cious predators of squid. If management objectives are 

 met and summer flounder and silver hake stocks are 

 fully rebuilt, predation on squid could rise substantially 

 as fish that have been functionally absent from the 

 population begin increasing in abundance. 



Other species, such as bluefish, that have been se- 

 verely overfished are also showing signs of recovery 

 (ASMFC'"*). Consumption of squid by bluefish has been 

 estimated to exceed the mass removed by the fishing 

 industry (Buckel et al., 1999b) and is contingent on 

 predator population abundance (Overholtz et al., 2000). 

 If one includes summer foraging habits, total consump- 

 tion of longfin squid by bluefish may be much greater 

 than previously estimated. In light of this new informa- 

 tion, a reassessment of the predatory demand imposed 

 by bluefish and other recovering predators on squid is 

 clearly needed before fish stocks are fully rebuilt. 



The simultaneous exploitation of predators and their 

 prey, when there is limited information about species 

 interactions, is a precarious practice that can have un- 

 intended and potentially detrimental consequences for 

 one or both stocks. Depletion of biomass that supports 

 higher predators may decrease biological production. 

 Conversely, prey populations may become overextended. 

 A greater understanding of species relationships is nec- 

 essary, especially in ecosystems where fishing occurs at 

 multiple trophic levels. The present study shows that 

 consideration of predation year-round and for a range 

 of predator sizes is imperative to accurately assess 



■> Cadrin, S. X. 2001. Status of fi.shery resources off the 

 northeastern United States: longfin inshore squid. Website: 

 http://www.nefsc.noaa.gov/sos/spsyn/iv/lfsquid/ laccessed 

 on 8 March 2006]. 



^2 Terceiro, M. 2003. Stock assessment of summer flounder 

 for 2003. Northeast Fisheries Science Center Reference 

 Document, 179 p. National Marine Fisheries Service, 

 Northeast Fisheries Science Center, 166 Water St.. Woods 

 Hole, MA, 02.543. 



^^ Brodziak, J. 2001. Status of fishery resources off the 

 northeastern United States: silver hake. Website: http:// 

 www.nefsc.noaa.gov/sos/spsyn/pg/silverhake/ laccessed on 

 8 March 20061. 



'••(ASMFC) Atlantic States Marine Fisheries Commission. 

 2003. Website: http://www.asmfc.org laccessed on 8 March 

 20061. 



